1. Field of the Invention
The invention relates generally to a digital intermediate frequency (IF) demodulator, and more particularly, to a digital IF demodulator that enhances the synchronization ability for TV systems.
2. Description of the Related Art
For traditional analog TV broadcast systems such as the National Television System Committee (NTSC), the Phase Alternating Line (PAL) and the Sequential Couleur Avec Mémoire (SECAM) systems, a baseband video and audio signal is modulated as an intermediate frequency signal based on corresponding modulation technologies specified for individual TV systems. The modulated IF signal is then frequency up-converted to a Radio Frequency (RF) signal via filtering, frequency up converting and signal amplifying. The produced RF signal is further transmitted through the atmosphere or by a coaxial transmission line. Using the NTSC system as an example, the baseband video signal is first sent to a Low Pass Filter (LPF) with the bandwidth of 4.2 MHz, and then output from the LPF with an output signal modulated as an IF carrier with frequency of 45.75 MHz in an Amplitude Modulation (AM) manner. Alternatively, the baseband audio signal may first be encoded using a Multi Channel Television Sound (MTS) mechanism and then sent to an LPF with the bandwidth of 200 KHz. Next, the output signal of the LPF is then modulated as an IF carrier with frequency of 41.25 MHz in a Frequency Modulation (FM) manner. Following, the video IF carrier is then mixed with the audio IF carrier, and the mixed signal is sent to a Band Pass Filter (BPF) having a 6.0 MHz bandwidth and 44 MHz central frequency. Finally, the BPF output signal is frequency up-converted and amplified as an RF frequency signal, and is then transmitted via an antenna or a coaxial transmission line.
At the receiving end, the received RF signal is typically coupled to a device, such as a tuner, in order to demodulate the received wideband RF signal with a specific RF frequency, such that the video and audio signals contained within a signal having a bandwidth of around 6 MHz carried by the RF frequency can be reconstructed. Most tuners are implemented by a superheterodyne architecture. For the receiving end, an IF demodulator is essential and traditionally implemented by an analog integrated circuit along with appropriate RLC (resistor/inductor/capacitor) elements. However, with advancement of System-On-Chip (SOC) technology, comprehensive and digital IF demodulators may be provided. SOC IF modulators have advantages such as lower cost and higher yield rates, and also reduce the effect of temperature/humidity variations, among other advantages, thus resulting in more stable IF modulator circuit performance.
IF demodulation generally comprises the steps of receiving an IF bandpass signal with a specific frequency, synchronizing and reconstructing the carrier signal from the IF bandpass signal, demodulating the signal modulated at the transmitting end, filtering the demodulated signal and separating the video and audio signals from the filtered signal, such that the baseband video and audio signals carried therein may be reconstructed. Among the steps, synchronization of the carrier signal is particularly important. This is because the quality of the reconstructed video and audio signals might be impacted if the carrier synchronization is not efficiently obtained, or a severe jitter is involved in the reconstructed carrier.
Apart from interference resulting from white noise, the Co-Channel Interference (CCI) resulting from multi paths as well as the Adjacent-Channel Interference (ACI) and the inter-modulation interference resulting from channel nonlinearity, are also common interferences found in analogy TV systems. In addition, in analogy TV systems, the carrier signal is transmitted in an in-band signaling manner. In this regard, apart from the interferences describe above, signals near the carrier signal (i.e. the luminance information of video signals) also affect the synchronization stability of the carrier signals. In addition, in some countries or systems, the frequency of the transmitted radio signal is adjusted in order to avoid frequency bands where interferences often occur. However, the adjustment results in a significant frequency shift for the receiving end IF frequency. If the carrier reconstruction mechanism of the IF demodulation system is not able to sufficiently adjust for high frequency shifts, reconstruction of appropriate video and audio signals will not be possible.